The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes using drilling equipment situated at the surface, and a drill string extending from the equipment on the surface to a subterranean zone of interest such as a formation. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling “mud” that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The mud exits the drill string via the drill bit and returns to the surface carrying rock cuttings produced by the drilling operation. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at the surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) that includes 1) the drill bit; 2) a steerable downhole mud motor; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) telemetry equipment for transmitting data to the surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars known as drill pipe. The MWD equipment is used to provide in a near real-time mode downhole sensor and status information to the surface while drilling. This information is used by a rig operator to make decisions about controlling and steering the drill string to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The operator can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation.
A drill string can comprise a downhole telemetry tool that contains a MWD sensor package to survey the well bore and surrounding formation, as well as telemetry transmitting means for sending telemetry signals to the surface, i.e. “uplinking”. Such uplinking telemetry means include acoustic telemetry, fibre optic cable, mud pulse (MP) telemetry and electromagnetic (EM) telemetry.
EM telemetry involves the generation of electromagnetic waves which travel through the earth's surrounding formations around the wellbore and to the surface. In EM telemetry systems, an alternating current is driven across a gap sub which comprises an electrically isolated joint, effectively creating an insulating break (“gap”) between the upper and lower portions of the drill string. An EM telemetry signal comprising a low frequency AC voltage is controlled in a timed/coded sequence to energize the earth and create a measureable voltage differential between the surface ground and the top of the drill string. The EM signal which originated across the gap is detected at the surface and measured as a difference in the electric potential from the drill rig to various surface grounding rods located about the drill site.
During a drilling operation, a drill operator can communicate with the downhole equipment by transmitting telemetry transmission from a surface transmitter to a downhole receiver in the downhole equipment. This operation is known as “downlinking” from surface and allows commands from the surface to be communicated to the BHA assembly. Various downlinking transmission means have been proposed, including transmission by EM. Downlinking by EM does present certain challenges. For example, EM downlinking, while advantageously not requiring mud flow to operate, can be significantly attenuated as EM signals travel through the Earth's formation, and high power is typically employed to ensure that EM signals reach a BHA that is far downstring. Providing a suitably powerful current source at the surface can present safety challenges, especially as the drill site can be a hazardous gas environment.